Copy over the sandbox APIs and the readme file #3
Description
Copy the API and readme from the MicroProfile sandbox location,
https://github.com/eclipse/microprofile-sandbox/tree/master/proposals/concurrency/
For the pom files, I am copying from existing MicroProfile projects so that we get the same level of validation.
The readme file will be copied identically, except with the addition of a license (required by the validation) and the addition of the gitter link.
The API files will be copied identically except for the removal of unused imports, which is required by the validation.